The present invention relates to a circuit configuration for controlling a servomotor by means of current pulses supplied by a current-controlled final stage. This circuit configuration is particularly suitable for controlling a servomotor of a control system for automotive vehicles, for example, the throttle valve-type servomotor of a traction slip control system.
A short circuit across the voltage source of a servomotor of the type under consideration (e.g. a short circuit across the power supply system of an automotive vehicle), during actuation of the motor through a current-controlled final stage, is difficult to identify because, with the short-circuited motor, the actual current value, as a result of the control, corresponds to the nominal value of the current. To identify a short circuit, instead of measuring the current, for example, the final stage voltage can be monitored. With the motor intact and upon actuation through a transistor, the voltage would fall to the saturation voltage of the transistor. When short circuit or shunt events occur within the motor, the voltage drop is substantially higher than in normal operation. However, the additional efforts involved with such a saturation voltage monitoring by no means are irrelevant.
German published application DE 33 18 909 C2 teaches an error-detection device for a motor control circuit which comprises a voltage divider, connected in parallel to the current source, and a comparator circuit. This comparator circuit monitors the voltage on a voltage divider point and generates an error signal once this voltage rises or drops to a value outside a predetermined voltage range as a result of an error (e.g. fusing of contacts, short circuit in the electronics).
To control the number of revolutions of a series-wound motor with a torque shutoff device which serves to drive a manually operated electric tool, a circuit configuration is described in German published application DE 38 19 166 A1 which switches off the motor upon the occurrence of a specific counter-torque. The motor current received is measured and evaluated as an indication of the issued torque.
A protection circuit device as disclosed in German published application DE 30 42 138 A1, which serves as an overload protection when a direct-current motor blocks, triggers a disconnection in the event that an excessive motor current is detected which lasts for more than a predetermined period of time.
Further, a device for the adjustment of the current of a battery to an electric motor is known from German published application DE 27 36 304 A1 which disconnects the current supply to the electric motor when a motor current pulse occurs.
Finally, a device for controlling the power of an electric consumer and comprising a short circuit monitoring means is described an German published application DE 37 27 498. For this purpose, the saturation voltage of a semiconductor switch is monitored which exceeds a predefined value once a short circuit occurs.